What is the point of using arrays of one element in ddk structures? - visual-c++

Question

Here is an excerpt from ntdddisk.h
typedef struct _DISK_GEOMETRY_EX {
DISK_GEOMETRY Geometry; // Standard disk geometry: may be faked by driver.
LARGE_INTEGER DiskSize; // Must always be correct
UCHAR Data[1]; // Partition, Detect info
} DISK_GEOMETRY_EX, *PDISK_GEOMETRY_EX;
What is the point of UCHAR Data[1];? Why not just UCHAR Data; ?
And there are a lot of structures in DDK which have arrays of one element in declarations.
Thanks, thats clear now. The one thing is not clear the implementation of offsetof.
It's defined as
#ifdef _WIN64
#define offsetof(s,m) (size_t)( (ptrdiff_t)&(((s *)0)->m) )
#else
#define offsetof(s,m) (size_t)&(((s *)0)->m)
#endif
How this works:
((s *)0)->m ???
This
(size_t)&((DISK_GEOMETRY_EX *)0)->Data
is like
sizeof (DISK_GEOMETRY) + sizeof( LARGE_INTEGER);
But there is two additional questions:
1)
What type is this? And why we should use & for this?
((DISK_GEOMETRY_EX *)0)->Data
2) ((DISK_GEOMETRY_EX *)0)
This gives me 00000000. Is it convering to the address alignment? interpret it like an address?

Answer 1

Very common in the winapi as well, these are variable length structures. The array is always the last element in the structure and it always includes a field that indicates the actual array size. A bitmap for example is declared that way:
typedef struct tagBITMAPINFO {
BITMAPINFOHEADER bmiHeader;
RGBQUAD bmiColors[1];
} BITMAPINFO, FAR *LPBITMAPINFO, *PBITMAPINFO;
The color table has a variable number of entries, 2 for a monochrome bitmap, 16 for a 4bpp and 256 for a 8bpp bitmap. Since the actual length of the structure varies, you cannot declare a variable of that type. The compiler won't reserve enough space for it. So you always need the free store to allocate it using code like this:
#include <stddef.h> // for offsetof() macro
....
size_t len = offsetof(BITMAPINFO, bmiColors) + 256 * sizeof(RGBQUAD);
BITMAPINFO* bmp = (BITMAPINFO*)malloc(len);
bmp->bmiHeader.biClrUsed = 256;
// etc...
//...
free(bmp);